Organic light-emitting diode (OLED) technology has been widely used to create digital displays in high resolution televisions, computer monitors, and handheld devices. The use of OLED technology provides advantages over other techniques, such as liquid-crystal display (LCD) technology, in various measures, such as energy efficiency, color contrast, resolution, and weight. A typical OLED display combines three basic colors—red, green, and blue—generated by electroluminescence of emitting materials.
Both fluorescent and phosphorescent materials have been developed as emitting materials. In principle, phosphorescent materials can achieve higher quantum efficiency than fluorescent materials because electroluminescent materials generate singlet-to-triplet excitation states in an intrinsic ratio of 1:3. Because of their relative increased quantum efficiency, noble metal-based phosphorescent molecules have been used as highly efficient and stable red and green light-emitting sources for OLED applications. However, phosphorescent materials that emit blue light have been found to be inefficient and unstable because of the highly energetic nature of excited states in blue light-emitting sources.
As an alternative, fluorescent materials minimize the lifetime of high-energy excited states, thereby resulting in a much faster light-emission process. However, as mentioned above, the intrinsic nature of electroluminescence limits the theoretical quantum efficiency of traditional fluorescent materials to 25%. Moreover, the other 75% of excited states (i.e., triplet excited states) can cause decomposition and, thus, undermine long-term stability of the material.
Thermally activated delayed fluorescence (TADF) can be used to address the efficiency and stability problems associated with fluorescent materials. TADF involves reverse intersystem crossing from triplet excited state to singlet excited state, which increases the theoretical quantum efficiency of fluorescence to a level comparable to phosphorescent materials. In addition, fast photo decay through the TADF process decreases the quantity as well as the lifetime of triplet excited states generated by excitons. To achieve TADF, a molecule must meet the standard general criteria for fluorescing, such as rigid structure to minimize non-radioactive decay, and also must have a small energy difference between its singlet and triplet excited states. Also, in order to achieve blue light emission, a compound must have a large energy gap between its highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
Despite some successes in developing red- and green-emitting TADF materials, a highly efficient and stable blue-emitting TADF source has not been discovered. There exists a need for a new class of stable, efficient, blue-light emitting compounds for OLED applications.